[WASM] Add minimal support for targeting wasm32-unknown-unknown-wasm. Forked #20684

[kateinoigakukun]

This PR is based on #20684 which adds the minimal wasm support.

And I added some test case to ensure that swiftc accepts wasm32-unknown-unknown-wasm and wasm64-unknown-unknown-wasm.

This is my first time to write lit test case, so please let me know if there is a problem.

[karwa]

In the time since the precursor to this PR was opened, our fork of LLVM has advanced enough to the point that it supports wasi. Rather than trying to support a "none" OS, why don't we switch to supporting the wasi triples (wasm32-wasi and wasm64-wasi) instead? That would seem to address @compnerd 's issues.

In platform-specific code, that's what you're going to be interested in, anyway:

#if os(wasi)
import WasiLibc // wasi's stripped-down libc

printf(...)

#endif

[kateinoigakukun]

I updated to use WASI as target OS

[karwa]

The original patch is now over a year old, then it finally became unarguably ready to merge, so of course someone had to immediately go and refactor that part of code so it now needs rewriting 😅

One day we will finally catch up to the likes of C, Forth and Lua, and support WASM.

[kateinoigakukun]

Hi, is there anything that prevents this PR from being merged?

[karwa]

@kateinoigakukun i believe @compnerd redid this after his changes and merged that version.

E.g. #28584

It would be great to rebase swiftwasm (there was another wasm-related patch, also from Saleem, after the one referenced above) based on master, and I think we can remove some hacks now that LLVM knows about WASI and that’s the „OS“ we support for WASM. I think @MaxDesiatov has write access to that repo.

I think we should really push ahead with upstreaming as many WASM patches as we can. This is a port that should be fairly easy to support via CI (basically all supported platforms can run WASM, even if it requires tools like wasmtime), and could be extremely useful for bootstrapping the Swift compiler if the driver project is ultimately successful at writing the compiler itself in Swift.